1. Field of the Invention
The present invention relates in general to a method of improving heat transfer in stored chemical energy powered coiled tube boiler and, in particular, to a method for enhancing the heat transfer performance of such a coiled tube boiler manufactured with a hydraulic expansion technique by providing fins or studs on the expanded coils.
2. Description of the Related Art
There are a variety of power sources operating from heat derived in the oxidation of metallic lithium, for example, U.S. Pat. Nos. 3,964,416 and 4,634,479. U.S. Pat. No. 3,964,416 converts this energy to steam to drive a turbine for propulsion of underwater vehicles. In such devices, it is desirable not to exhaust the products of combustion into the sea.
The Stored Chemical Energy Propulsion System (SCEPS) as disclosed therein employs a lithium-fueled boiler which supplies steam to a turbine. The turbine is connected to a gearbox that drives the propulsor. The boiler consists of two helical coils, an inner and an outer coil, arranged to provide an annular cylindrical cavity for the lithium fuel. Each helical coil is fabricated from stainless steel tubing that is coiled and welded to form the inner and outer containment walls of the boiler. The heat source in the boiler is a result of an exothermic chemical reaction between lithium fuel and injected sulfur hexafluoride (SF.sub.6) which acts as the oxidant. The heat generated by the exothermic reaction is transferred from the lithium-fuel side of the boiler to the inside of the tubing and converts feedwater into steam.
Hydraulic expansion manufacturing techniques are known for creating flow channels. U.S. Pat. No. 4,295,255 issued to Weber describes a method of manufacturing a cooling jacket assembly for a control rod drive mechanism. This technology has further been applied to creating a flow channel as depicted in FIG. 1 and is referred to hereinafter as a coiled-tube boiler. The flow channel finds particular utility for both the inner and outer helical coils of the SCEPS boiler as depicted in FIG. 7. To fabricate a flow channel (inner or outer helical coil), one cylinder (12) is placed inside another cylinder (14) and an electron beam welder (not shown) spirally welds in a helical weld path (16) the two cylinders (12, 14) together. After welding, hydraulic pressure is applied between the welds (16) of the two cylinders (12, 14). As the hydraulic pressure increases, the cylinders (12, 14) deform between the helical weld paths (16) creating a flow channel (18) as is illustrated in FIG. 1.
It is also known in the art that internal ribs in tubes increase heat transfer performance as disclosed in U.S. Pat. Nos. 3,088,494 and 4,044,797. These ribs are provided in the tubes after the tube is formed by milling, machining, drawing or swaging processes known in the art.
There are tubes of very hard materials such as Inconel 625 which are extremely difficult to provide ribs in. Also, it can be very costly to form ribs in long sections of tubing. Moreover, in small diameter tubing, it is difficult and sometimes not even practical to form ribs therein.
U.S. patent application Ser. No. 07/666,276 filed Mar. 7, 1991 which is assigned to the Assignee of the present invention and hereby incorporated by reference discloses a method for forming ribs in a coiled tube boiler.
During operation of the SCEPS boiler, by-products of the lithium reaction deposit and freeze on the outer and inner coils when the metal temperature is below about 1450.degree. F. This creates an insulating layer for the coil which decreases the thermal output of the boiler as the operation proceeds.
Studs (2) have been employed on tube membrane walls (3) in conventional boilers for holding a refractory (4) in place on the hot side as shown in FIG. 2. The studs or pins (2) are welded to the tubes (3) at close intervals and covered with a slag-resistant refractory material. This construction increases furnace temperature to maintain the coal, peat, or lignite ash in a liquid state.
It is desirable to improve heat transfer in a stored chemical energy powered coiled tube boiler and other hydraulically expanded heat exchangers. The method should allow for a rapid and simple fabrication of studs or fins in hard materials and on small nonuniform diameter tubing.